Human placental alkaline phosphate is an isoenzyme which appears to be unique to normal fetal development. A similar isoenzyme appears in cancer which may or may not be a product of the same gene as the placental enzyme. To compare the structures of the enzymes appearing in cancer and pregnancy, immunochemical techniques are being applied to the detailed structures of the enzymes in placental and cancer tissues. The specific immunoreactivity of these enzymes using rabbit, sheep, monkey and chicken antisera is being studied. To date, antigenic sites have been identified that are specific for individual genetic variants, and differences have been found in the immunoreactivity of membrane-bound and soluble enzyme.